Because of their excellent chemical resistance, thermal resistance and mechanical properties, polytetrafluoroethylene resins (hereinafter abbreviated sometimes to PTFE) are used in various fields. For instance, porous membranes made of PTFE are widely used, utilizing such properties as referred to above, as filters for corrosive materials or high temperature substances, and also used as diaphragms for electrolytic baths, fuel cells, etc.
In recent years, moreover, with the developments in the semi-conductor industry and in molecular biology, porous membranes having pores which are of nearly round and uniform in pore diameter are in demand for the purpose of removing very minute impurities from gases or liquids. PTFE porous membranes as characterized above have come to attract keen interests of various fields of industry.
For the production of porous membranes from PTFE resins, there has heretofore been adopted a process in which PTFE finely divided particles, present as fine powders having an average particle diameter of 0.1-0.4 .mu.m obtained by emulsion polymerization of tetrafluoroethylene, is incorporated into a liquid lubricant, and the mixture is compression molded to prepare it to a preform. The preform is then processed into a film form by an extruding or rolling technique or combination thereof, the liquid lubricant is removed therefrom, and the PTFE membrane obtained is monoaxially or biaxially stretched under the usual heating conditions. For instance, Japanese Patent Publication No. 42794/1978 discloses a process for the production of PTFE porous materials, wherein the sintered PTFE membrane is heat treated at a temperature above 327.degree. C., followed by slow cooling, so that crystallinity of PTFE in the membrane becomes 80% or higher, and the membrane, thus treated, is stretched at a temperature of 25.degree.-260.degree. C. monoaxially at a draw ratio of 1.5-4 times.
However, the process referred to above involved difficulties in that because the starting material used is a PTFE fine powder, the PTFE membrane prepared therefrom is liable to be fibrillated at the time the membrane is stretched, and hence it is difficult to produce PTFE membranes having pores which are nearly in a round shape and a predetermined pore diameter. That is, the pores formed in the PTFE porous membranes obtained by this process were of a long elliptical form having a marked difference between the major axis and the minor axis thereof, and hence it was difficult to obtain pores nearly of a round shape even when the PTFE membranes were biaxially stretched. Furthermore, this process involved such problems that the mechanical strength of the porous membranes obtained thereby cannot be said to be sufficient.
We have conducted researched in an effort to solve the above-mentioned problems and have eventually accomplished the invention on the basis of the following facts. That is, it has been discovered that (a) in a process for production PTFE porous membrane, it is better to use a molding power of PTFE obtained by suspension polymerization of tetrafluoroethylene than the use of finely divided particles of PTFE known as the aforesaid fine powder obtained by emulsion polymerization of tetrafluoroethylene. A further discovery is that (b) in a process for stretching PTFE in a film form under heating conditions, by virture of the addition thereto of such an operation that the PTFE in a film form is previously heated at a temperature above 327.degree. C. and then slowly cooled at a cooling rate of less than 70.degree. C./hr so as to adjust the crystallinity of PTFE to 60-75%, a PTFE porous membrane having greatly improved pore characteristics and mechanical strength as well as an excellent porosity can be obtained.